1. Field of the Invention
The present invention relates generally to Belleville or disc springs. More particularly, apparatus and method are provided for decreasing friction and hysteresis effects in operation of the springs.
2. Background of the Invention
Belleville Springs are conical shaped circular springs that were patented in 1867 by Julian Belleville. They are also called “Belleville Washers,” because in plan view they have the shape of a washer. They are also called “disk springs.” Subsequent improvements have advanced this simple spring device to a sophisticated energy storage system that is used in many mechanical systems today. The springs are designed to be loaded in the direction perpendicular to the washer, i.e., by compressing the cone, and they may be loaded statically or dynamically. Disk springs are used, for example, in brakes, clutches, valves, shock absorbers, actuators, loading of flange joints, and in a variety of mechanical equipment for use in wells, such as jars, accelerators, shock subs, clutches, drilling motors and other equipment.
Belleville springs (disk springs) are available in a wide range of sizes, from about 8 millimeters to about 800 or more millimeters. When multiple springs are used together, they are usually used with a mandrel inside the springs or a cylindrical surface in contact with the outside periphery of the springs to serve as a guide, so they can be maintained in alignment when a load is applied. They may be delivered pre-assembled in stacks with the guiding device. Combinations of disks having different spring characteristics may be combined in a stack to produce a wide variety of load-deflection curves. The disks may be selected to provide specific load-deflection diagrams. Individual disks may be externally slotted or internally slotted to produce a load-deflection characteristic for specific applications. The disc blanks may be manufactured by stamping, fine blanking or plasma arc/flame cutting and they may receive a variety of metallurgical treatments.
Design equations are readily available for any combinations of disk springs. Stacks may be made up of springs in parallel or series configuration. In the parallel configuration, springs are arranged in parallel, i.e., with the cone in the same direction. In the series configuration, the external circumference of springs is in contact with that of an adjoining spring and the internal circumference is likewise in contact with that of an adjoining spring. It is possible to generate characteristic curves for stacks of springs by combining parallel stacks with a selected number of disc springs and series configurations of other springs. A manual published by Mubea Tellerfedern und Spannelemente Gmbh of Daaden, Germany, provides information on design theory and properties of Belleville springs and stacks of Belleville springs.
In addition to disc spring stacks having a mandrel or cylinder as a guide, self-centering disc spring stacks are available. These may be centered by balls and grooves on the inner and outer diameter. The self-centering is primarily used on stacks of large disc springs. Wire-centered disc spring stacks have also been used, substituting wire segments for the steel balls. Friction is slightly higher with this type centering. A T-ring or retaining ring may also be used for centering disc spring stacks.
It has long been known that friction is important during operation of Belleville springs. The friction may arise between individual springs, between the springs and the guide element, and at the edges of the spring where load is applied. This friction results in a variation between the calculated characteristic load curve and an actual load curve. The force required to compress the springs is greater than the force recovered when the springs are relaxed, which means that a load-deflection diagram exhibits hysteresis. The area within a hysteresis loop is indicative of the effect of friction on operation of the spring system.
One of the sources of friction in operation of a Belleville or disc spring is the friction between the central and outside peripheries of the spring and the load surfaces coming in contact with those faces. U.S. Pat. No. 3,261,598 discloses mechanisms to minimize the frictional losses from radial movement of a Belleville washer between two flat surfaces. Spring supports that may be deflected with minimal force in a radial direction are employed. The spring supports may be applied at both the central opening and the outside periphery of a Belleville spring.
U.S. Pat. No. 3,375,000 addresses a different source of frictional losses in stacks of Belleville springs. It discloses a Belleville spring assembly for reducing surface friction between springs that are mounted in a parallel stacked array. A bearing element between the inner and outer edges of adjacent discs is employed. The patent states that stacked arrays of parallel discs in abutting relationship produce undesirable hysteresis losses as a consequence of friction imposed by contacting surfaces through flexure of the washers and associated parts.
U.S. Pat. No. 3,873,079 discloses a number of Belleville spring discs held in coaxial relation by inner and outside split retainer rings that are expansible by the springs when the springs are deflected axially and spread at their periphery. The retainer rings have v-shaped grooves for receiving the edges of the springs while affording a clearance space to minimize friction during deflection of the springs.
U.S. Pat. No. 5,081,328 describes one of the wide varieties of applications of Belleville springs—the use in a switch. The patent also describes the limitations of Belleville springs because of frictional effects, explaining that the motion required to convert fluid pressure to mechanical work results in Belleville springs bending, sliding, scraping and plowing at the inside diameter or outside diameter of the springs. The result of friction increases hysteresis and causes an increase in a switch's “deadband” (the difference between the point of operation and the point that it returns to its pre-operated state). The '328 patent discloses avoiding the difficulty of the Belleville springs (because of frictional effects) by using a pressure-sensing negative rate membrane.
In stacks of springs guided by a rod or mandrel, springs may be designed with a special inner edge contour in an attempt to minimize friction between the guide rod and the disc springs. However, deviations in geometry of individual disc springs result in an uneven transmission of load from one spring to the next in a stack. This results in forces tending to cause a lateral displacement of the springs, or a buckling of the stack, causing the springs to be pressed with force against the guide element. This lateral force is similar to a buckling force observed in a rod or tube with ends under compression. Thus, there is a need for apparatus and method to minimize the net frictional effects arising from lateral forces or buckling. This will reduce the overall effect of friction on operation of stacks of Belleville springs, whether the springs are guided by a mandrel or a sleeve.
When Belleville springs are used in downhole jars, for example, friction force on the springs prevents a constant triggering load of the jar. This is discussed in Pub. No. U.S. 2005/0092495, at page 6, col. 1. The referenced publication is hereby incorporated by reference herein in its entirety.
It is sometimes necessary to include a stroke-limiter with a Belleville spring. For example, if the spring is deflected beyond a certain limit, it may reverse direction, or excess deflection may cause permanent change in spring characteristics. Therefore, there is a need for apparatus to prevent excessive deflection of a Belleville spring along with the reduction of the effects of friction on operation of stacks of Belleville springs.